The present invention relates to an index value calculation method, a satellite signal selection method, a satellite appropriateness calculation method, a calculation circuit, and a positioning device.
The global navigation satellite system (GLONASS), the GALILEO, the quasi-zenith satellite, and the like are known as satellite positioning systems. The most widespread satellite positioning system is the global positioning system (GPS).
According to the GPS, four GPS satellites are disposed in each of six earth orbits at a tilt angle of 55 degrees, and are operated so that each GPS satellite orbits the earth every 11 hours, 58 minutes, and 2 seconds. Therefore, the satellite constellation of the GPS satellites changes depending on the present position and the positioning time. A dilution of precision (DOP) is known as an index which indicates the effects of the satellite constellation on the positioning accuracy. The DOP is a numerical value which indicates the satellite constellation of each GPS satellite when observing the sky from a point (present position) on the ground. In other words, the DOP is a numerical value which indicates the degree by which the satellite constellation of each GPS satellite decreases the positioning accuracy. Various technologies of accurately locating a position using the DOP have been developed (see JP-A-2006-112936, for example).
However, the positioning accuracy is not affected by only the satellite constellation. For example, the positioning accuracy is affected when a person carrying a GPS positioning device enters a building, or is affected by a multipath. Specifically, the positioning accuracy varies to a large extent depending on a reception environment in which a satellite signal is received. Therefore, it is convenient if an overall index taking into account the satellite constellation and the reception environment can be calculated. Since the DOP is an index of the entire satellite constellation, it is convenient if a relative index of each satellite in the present satellite constellation can be calculated.